Radioluminescence may be defined as the generation of light through the excitation of a phosphor by radiation, such as beta or gamma radiation, resulting from the decay of a radioactive element. In radioluminescence, the excitation of the phosphor occurs by the bombardment of the phosphor with subatomic particles or rays from the radioactive element. One of the first applications of radioluminescence was in luminescent paints for use on watches, clocks, aircraft instrument panels and similar devices. A common formulation for these radioluminescent paints was a mixture of radium and a zinc sulphide phosphor. While these paints were quite luminescent, their use was stopped once the toxicity of radium was recognized. This led to the development of radioluminescent paints containing less harmful radioisotopes, such as tritium. Generally, tritium was incorporated into these paints by substitution for hydrogen in the organic resins of the paint, which was used as a binder for the zinc sulphide phosphor. Because of the opacity of the resin, and the tendency of tritium to desorb out of the resin binder, these paints were, at best, inefficient light sources.
A further type of radioluminescent light source, developed in the 1960's, was tritium gas filled glass tubes. In these lights, the interior surface of the tube was coated with a phosphor, such as zinc sulphide, and the tube was filled with tritium gas. While these lights are much more efficient than radioluminescent paints, commercial acceptance of these lights has been hampered, due to the public perception of dangers resulting from possible breakage of the lights.
A solid state radioluminescent light source is shown in U.S. Pat. No. 5,118,951 to Kherani et al. This patent discloses a light source comprising a radioactive element entrapped in an amorphous semiconductor. While this device has some advantages over conventional radioluminescent paints and phosphor coated tritium gas filled glass tubes, it does have several limitations, as detailed below.
In a semiconductor material, excitation of the material by an external source creates an excess of electron-hole pairs above the number of electron-hole pairs present at thermal equilibrium. The excess carriers then recombine to discharge their energy and return the semiconductor to equilibrium. The main mechanisms for this recombination are radiative recombination, in which the recombination process results in light emission, and non-radiative recombination, in which the recombination results in heat emission. The efficiency of a luminescent semiconductor is a measure of the fraction of the total number of excess carriers which radiatively recombine to produce light.
The rate of radiative recombination is dependent on the carrier lifetime, which determines the number of excess carriers that can form electron-hole pairs, and the pair recombination coefficient, which expresses how likely it is for a given pair to recombine radiatively. Amorphous semiconductors typically are characterized by short carrier lifetimes and low recombination coefficients, and therefore have limited useful light output. Accordingly, devices incorporating amorphous semiconductors have limited commercial applications.
Further, as the non-radiative, or heat producing, recombination mechanism of amorphous semiconductors is very efficient, the radiative efficiency is further lowered. In amorphous semiconductors, there are typically densities of non-radiative recombination centers that are at least as high as the densities of the radiative recombination centers, and often higher. This effect also contributes to limiting any spectral output of amorphous semiconductor light sources to infrared.
A further limitation of using amorphous materials for light emitting applications is their notorious instability under even weak excitation due to the Staebler-Wronski effect. The underlying mechanism for this is defect formation due to the easy breaking of weak bonds, resulting in rapid material damage which prevents obtaining useful light emission under radioisotope excitation.